Size-dependent transdermal absorption of cadmium sulphide quantum dots: an ex-vivo study using Franz diffusion cells

Cadmium sulphide (CdS) quantum dots (QDs) are semiconductor nanomaterials extensively used in optoelectronic and biomedical applications, raising concerns regarding potential health hazards associated with dermal exposure. This study investigated the size-dependent transdermal absorption of N-acetylcysteine (NAC)-capped CdS QDs using an ex-vivo human skin model and Franz diffusion cells. Three QD sizes (3.4, 5.8 and 6.7 nm) were synthesised in aqueous solution, characterised by UV-Vis and fluorescence spectroscopy, and applied to intact and damaged human skin. Cadmium permeation into the receptor fluid and accumulation within skin layers were quantified by ICP-MS, while QD distribution was qualitatively assessed by fluorescence microscopy. Intact skin effectively limited CdS QD permeation, with cadmium levels comparable to blank controls and predominant retention within the epidermis. In contrast, damaged skin exhibited a marked and statistically significant increase in cadmium penetration, strongly dependent on particle size. The smallest QDs (3.4 nm) showed the highest permeation and tissue accumulation, reaching 303 ± 135 μg/cm² after 24 h, followed by 5.8 nm QDs, whereas 6.7 nm QDs displayed minimal penetration. Fluorescence imaging confirmed enhanced epidermal and trans-epidermal localisation of smaller QDs, particularly under compromised barrier conditions. Overall, these findings demonstrate that nanoparticle size and skin barrier integrity are key determinants of CdS QD dermal absorption. The results provide relevant evidence for hazard identification and risk assessment of cadmium-based nanomaterials, especially in occupational and environmental exposure scenarios involving impaired skin integrity.